Conventionally, there is known a structure for cooling an engine body by coolant fed from a coolant pump.
For example, Japanese Patent No. 5,223,389 discloses a cooling structure in which coolant fed from a coolant pump flows into an engine body, a part of coolant whose temperature is increased by cooling the engine body is returned to the coolant pump via an EGR cooler and a heater, and the coolant is fed to the engine body again.
In the structure disclosed in Japanese Patent No. 5,223,389, after all coolant fed from the engine body passes through heat exchangers such as an EGR cooler and a heater, and is warmed or cooled by the heat exchangers, the coolant is fed to the engine body again. In the aforementioned configuration, there is a problem that a temperature of coolant fed to the engine body is likely to vary depending on an amount of heat exchange in the respective heat exchangers, and a cooling state of a combustion chamber formed in the engine body is not stabilized. Further, accompanied by the unstable state, a combustion state of fuel-air mixture within the combustion chamber may also become unstable.